Molecular characterization of circulating tumor cells

ABSTRACT

The disclosed invention includes methods and kits for the removal of white blood cells from samples of enriched rare cells.

RELATED APPLICATIONS

This application claims priority to a pending provisional patent application, U.S. Ser. No. 61/787,611, entitled “Improved Molecular Characterization of Circulating Tumor Cells” which was filed on Mar. 15, 2013

BACKGROUND

Circulating tumor cells (CTC) of epithelial origin are present in the blood of carcinoma patients at very low frequency (<10/ml blood). The detection of tumor cells in circulation may have significance for cancer disease management. The detection of low frequency cells requires a large blood volume for processing. In order to enumerate and characterize CTCs from large blood volumes, the enrichment of CTCs is necessary.

Several methods are available to enrich CTCs based on size, density and antigen. The well established commercial product for enrichment of rare cells is Veridex CellSearch CTC assay. The CellSearch CTC assay uses magnetic particles conjugated to anti-epithelial cell adhesion molecule (EpCAM) to capture CTCs from 7.5 ml of blood. The enriched samples are stained with DAPI, a nucleic acid dye, to identify nucleated cells, anti-cytokeratin antibodies conjugated to phycoerythrin in order to identify cells of epithelial origin, and anti-leukocyte antibodies conjugated to allophycocyanin to identify all leukocytes. The samples are analyzed on a CellTracks Analyzer II for enumeration of CTCs.

The final sample after the enrichment contains CTCs and a small number of white blood cells (1000-5000 cells). The enrichment method removes more than 99% of white blood cells (WBC). The presence of white blood cells during the enumeration of rare cells is not an issue does not adversely affect the process. WBCs may be labeled with a leukocyte marker (CD45) to differentiate WBCs from CTCs. However, if one wants to molecularly characterize the enumerated CTCs to define their genotype by amplification of the nucleic acids present in such CTCs the presence of white blood cells in adversely affects this characterization. Given that the ratio of white cells in an enumerated fraction is high compared to the number of CTCs and the nucleic acids of such white cells is amplified with the CTCs it is difficult was difficult to determine the source of the nuclei acids, namely CTCs or white blood cells. As a result, further purification of enriched samples to remove white blood cells (decrease the ratio of white blood cells to CTCs) would be a major milestone for developing a reliable molecular characterization tools. This need is met by the following invention.

There are no methods available to remove white blood cells after the enrichment of CTCs. It requires a unique method since the same principle used for the enrichment of CTCs will not be used. For example, if the enrichment method in the first step uses the immunomagnetic method, another method besides the immunomagnetic method must be used.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 Depiction of hapten based white blood cell removal

FIG. 2 Analysis of white blood cells before and after incubation with anti-FITC

FIG. 3 Percentage white blood cell removal

FIG. 4 Molecular measurement of depleted and non-depleted samples

FIG. 5 Detection following CD45 depletion

FIG. 6 Correlation between spiked-in Vcap cells with shite blood cell depletion and cultured Vcap cells

FIG. 7 Effect of bead depletion.

DETAILED DESCRIPTION OF INVENTION

An invention described herein comprises, consists essentially of or consists of a method of removing white blood cells from a sample comprising enriched rare cells and white blood cells such method comprises

-   -   (a) treating such sample with a leukocyte marker that is         conjugated to a hapten under conditions that label such white         blood cells with such leukocyte marker conjugated to a hapten     -   (b) treating the composition of step (a) with a second medium         that adheres to such labeled white blood cells, and separating         the second medium and its adhered labeled white blood cells from         the composition of step (b).

As used herein “leukocyte markers” refer to substances that label white blood cells and not rare cells, Examples of leukocyte markers include but are not limited to antibodies to CD 45, CD 19 CD15, glycophorin A, CD2, CD14. CD16, CE38, and CD66b. The preferred leukocyte marker is anti CD45. Haptens include but are not limited to fluorescein dye (“FITC”) phycoerythrin (“PE”), allophycocyanin (“APC”) and biotin. The preferred hapten is FITC. As used herein the term antibodies includes whole antibodies both monoclonal and polyclonal, antibody fragments that bind with the certain haptens, bi-specific antibodies that bind to certain haptens. The preferred antibodies are monoclonal antibodies.

As used herein, “second medium” means any surface comprising a second marker, other than a leukocyte marker. Such second marker binds to the hapten that is conjugated to the leukocyte marker. Examples of second markers include but are not limited to antibodies to FITC, PE, APC, the preferred second markers are antibodies to FITC. Examples of surfaces that may be used are microtiter plate

As used herein “rare cells” are cells that have a low frequency in blood. Examples of rare cells include but are not limited to circulating tumor cells (CTCs), circulating endothelial cells (CECs) circulating multiple myeloma cells (CMMCs) and circulating melanoma cells(CMCs). The preferred rare cells are CTCs and CECs, the particularly preferred rare cells are CTCs. Enriched fractions of these rare cells may be produced from whole blood by known methods. Such methods include but are not limited to the methods and reagents disclosed in the following patents and patent applications: U.S. Pat. Nos. 7,901,950; 6,365,362, US Pat. Pub. Nos. US 2009/0136946; US 2013/0189675; US 2014/0011685 which are hereby incorporated by reference in their entirety. The preferred method of obtaining enriched CTCs is by the immunomagnetic methods disclosed in references such as U.S. Pat. No. 6,365,362 and by the methods of the CELL SEARCH product line.

The principle of depleting white blood cells from an enriched fraction of CTCs. The CTCs and the white blood cells are labeled with an antiEpCAM ferrofluid and the white blood cell is labeled with a leukocyte conjugated hapten is illustrated in FIG. 1.

Further the invention includes a kit for removing white blood cells from a sample comprising enriched rare cells and white blood cells wherein such kit comprises a leukocyte marker that is conjugated to a hapten and a second marker, other than the leukocyte marker wherein the second marker comprises antibodies to such haptens.

All of the defined terms have the same definitions and preferred examples as described above.

Still further the invention includes a kit for removing white blood cells from a sample comprising enriched rare cells and white blood cells wherein such kit comprises reagents for immunomagnetically marking the rare cells and white blood cells, a leukocyte marker that is conjugated to a hapten and a second marker, other than the leukocyte marker wherein the second marker comprises antibodies to such haptens.

All of the defined terms have the same definitions and preferred examples as described above.

The invention is illustrated by the following examples, which are not meant to limit the invention's scope.

EXAMPLES Example 1 Depletion of White Blood Cells Using Anti CD45 Coated on a Microtiter Plate

This example shows the depletion of white blood cells using anti CD45. CD45 is a common leukocyte marker present on white blood cells and it is expected that all white blood bind to anti CD45. We used anti-CD45 to bind white blood cells and specifically remove them from a mixture of white blood cells and CTCs using a solid phase coated with anti CD45. Anti-CD45 was coated to a solid phase as follows:

Anti-CD45 (Veridex) was diluted to 50 ug/ml in 50 mM sodium bicarbonate buffer pH 8.5. 0.8 ml of anti-CD45 antibody solution was then added to 6 well microtiter plate, incubated for 3 hours at room temperature (RT) followed by overnight at 2-8° C. After overnight incubation, anti CD45 antibody was aspirated and the wells were blocked with 1 ml of PBS/1% BSA at RT for 4 hours. The buffer was aspirated and the wells were rinsed with 2 ml of PBS 2-times. After the rinse, the entire buffer was aspirated and the plate was dried for 1 hour. The plate was stored dry in a sealed plastic bag at 2-8° C. until use.

In this example, two experiments were done to test the principle of separating white blood cells from non-white blood cells. In one study, pure white blood cells were prepared from whole blood after lysing red blood cells using BD lysing reagent. This sample was used as a positive control. In another study, 7.5 ml of blood was processed on the CellTracks AutoPrep system using CellSearch CTC Profile kit. The CTC Profile kit contains anti-epithelial cell adhesion molecule (EpCAM) conjugated to ferrofluid magnetic particles for the capture of circulating tumor cells. After the enrichment of target cells, the samples were resuspended in 900 ul of PBS/1% BSA buffer. The enriched samples will contain target cells and also white blood cells. Before samples were added to the wells in the microtiter plate, the plate was brought to room temperature (minimum of 30 minutes), the wells were washed twice with 2 ml of PBS/5% BSA. The buffer from the wells was then aspirated before the sample was added. The 900 ul CTC assay sample and pure white blood cells from whole blood were added to two separate wells of the plate. Following a 1 hour incubation with gentle mixing every 15 minutes, 300 ul of supernatant was removed to determine the numbers of white blood cells. The number of white blood cells present in the sample was determined by FACSCalibur flow cytometer (Beckton Dickinson) using forward scatter as a threshold. If the white blood cells bind to anti-CD45 in the wells then the number of white blood cells in the supernatant should be decreased when compared to the sample before the incubation step. The unbound cells should be in the supernatant. The results are shown in Table 1a.

TABLE 1a White blood cell numbers before and after incubation with anti CD45 coated onto a microtiter plate WBC # Before After incubation with incubation with anti-CD45 anti-CD45 % WBC Sample coated plate coated plate Removal White blood cells 1000 200 80 without CTC assay CTC assay sample 1200 1300 0

The above results show that pure white blood cells can be removed by adding them to the microtiter plate coated with anti-CD45. However, white blood cells were found in the supernatant of the CTC assay sample. This suggests that white blood cells from the CTC assay sample did not bind to anti-CD45 coated in the wells. The difference between the two samples is that white blood cells in the CTC assay sample are labeled with ferrofluid magnetic particles. It is possible that ferrofluid magnetic particles present on white blood cells are preventing binding to anti-CD45 on the plate due to steric hindrance. To overcome this problem, the samples from the CTC assay were first labeled with anti-CD45 conjugated to a hapten by a linker (See Examples). The samples containing white blood cells prelabeled with the anti-CD45-tag were then added to a microtiter plate coated with anti-hapten.

Example 2 Coating of Anti-Fluorescein Isothiocyanate (FITC) to the Microtiter Plate

Anti-FITC was purchased from BD Biosciences and diluted to 50 ug/ml in 50 mM sodium bicarbonate, pH 8.5. 800 ul of anti-FITC was added to 6 well microtiter plates, incubated at RT for 3 hours followed by overnight at 2-8° C. After overnight incubation, the plate was brought to RT. The supernatant was aspirated and then the plate was rinsed twice with PBS. The wells were then blocked with 1 ml of PBS/1% BSA for 4 hours at RT. The buffer was aspirated and the wells were rinsed with 2 ml of PBS 2-times. After the rinse, the entire buffer was aspirated and the plate was dried for 1 hour. The plate was stored dry in a sealed plastic bag at 2-8° C. until use. The plate was brought to RT (minimum of 30 minutes) on the day of use. 2 ml of PBS/1% BSA was added to the wells and incubated for 15 minutes. Following aspiration the plate was once again rinsed with 2 ml of PBS/1% BSA. The buffer was aspirated before sample was added to the well.

Example 3 Removal of White Blood Cells from CTC Assay Sample Where White Blood Cells are Labeled with Anti-CD45 Conjugated to FITC and Microtiter Plate Coated with anti-FITC

7.5 ml of EDTA blood spiked with SKBR3 cells was processed on the AutoPrep using CellSearch CTC kit. This enriched sample from the CTC assay was stained with anti-CD45-FITC at a final concentration of 2 ug/ml. The excess CD45-FITC was removed by washing the sample 2-times with 2 ml of PBS/1% BSA by magnetic separation for 15 minutes. The final sample was resuspended in 900 ul of PBS/5% BSA and then applied to the wells coated with anti-FITC. The samples were incubated for 1 hour with gentle mixing every 15 minutes. After one hour, 300 ul of supernatant was removed from the wells and the number of white blood cells and SKBR3 cells were determined by flow cytometer. To detect SKBR3 cells by flow cytometry, the cells were labeled with anti-Her2neu conjugated to allophycocyanin dye (APC). The white blood cells were detected in the FITC channel since they were labeled with anti-CD45-FITC. The cells detected before and after depletions are shown in FIG. 2.

FIG. 2 shows the number of tumor and white blood cells before and after depletion of white blood cells. The number of white blood cells before and after depletion are 3594 and 354, respectively which indicates that approximately 90% of white blood cells are depleted from the sample. On the other hand, the number of tumor cells before and after depletion are 2094 and 1924, respectively which indicates that all the tumor cells (>90%) are present in the supernatant. This example shows that white blood cells present in the CTC assay sample after tumor cell enrichment can be removed using the invention

This was tested with on 6 samples and the results are shown in FIG. 3. The results show that more than 90% of WBCs can be removed with minimal loss (<15%) of tumor cells (SKBR3).

Example 4 Depletion of White Blood Cells in Circulating Endothelial Cell (CEC) Assay

This example shows that the principle of WBC depletion can be applied to other rare cell assays. The CellSearch CEC assay enriches CECs from blood using CellSearch CEC kit. The kit contains anti-CD146 conjugated to ferrofluid magnetic particles for the capture of CECs. The assay uses CellTracks AutoPrep system for the sample preparation much like the CTC assay. While CECs are present at a low frequency in normal healthy blood samples, they are elevated by various conditions such as cancer, cardiovascular problems and infection.

The assay was used to test the effect of the depletion principle on CECs present at low a frequency (1-20 cells per test) in addition to WBCs depletion efficiency.

4 ml of EDTA blood was processed on the CellTracks AutoPrep system using CellSearch CEC assay. The enriched cells were stained with nucleic acid dye (DAPI) and anti-CD105 conjugated to phycoerythrin dye (PE) to identify all cells and CECs, respectively. In this example, anti-CD45-FITC was used to label white blood cells. After the staining step, the sample was resuspended in 320 ul of buffer and transferred to a CellSearch analysis cartridge for analysis on the CellTracks Analyzer II. The CellTracks Analyzer II is a 4-color fluorescent microscope which scans in 4 different colors, analyzes images and presents images which are positive in DAPI and CD105-PE. The cells which are positive for DAPI and CD105 and negative for CD45 are counted as CECs. The total numbers of WBCs present in the sample were counted based on DAPI positivity.

For the depletion step, following enrichment and staining, the samples were resuspended in 900 ul of PBS/1% BSA. The samples were then added to the microtiter plate coated with anti-FITC as described in example 4. After one hour, the sample was aspirated and concentrated to 320 ul using a magnetic separator. The sample (320 ul) was then transferred to a cartridge for analysis on CellTracks Analyzer II.

The number of CECs and WBCs were determined as described above. The number of CECs and WBCs were compared before and after depletion of WBCs. The results from this study are shown in the Table 4a.

TABLE 4a Number of CECs and WBCs before and after depletion of WBCs CECs# WBC Sample Before WBC After WBC WBC# Depletion # Depletion Depletion Before Depletion After Depletion (%) D587 3 3 11,877 1,499 87.4 D598 2 6 12,116 3,663 69.8 D599 4 4 3,463 less than 300 NA D611-1 17 15 12,835 2,694 79.0 D611-2 17 12 12,427 3,307 73.4 Ave. 8.6 8 10544 2791 77.4

The results clearly show that WBCs can also be removed (>75%) from CECs after enrichment in the CEC assay. In addition, there was no difference in CECs numbers (8.6 vs. 8) before and after depletion. The data indicates that the target cells are not removed even when they are present at very low frequency (<20 cells).

Example 5 Molecular Measurements of WBC Depletion in Cell Search Profile Kit Enriched Samples

Duplicate 7.5 ml of EDTA blood from 6 healthy donors was processed on the AutoPrep using a modified CellSearch CTC Profile Kit. The PBS/Biotin reagent in the Profile Kit was supplemented with anti-CD45-FITC at a final reagent concentration of 2 ug/ml so that labeling of WBC takes place on the AutoPrep. Following AutoPrep enrichment, one tube from each donor was subjected to either the “Depleted” protocol which removed the WBC from the sample or the “Non-depleted” protocol which serves as a control to measure the WBC contamination in the sample without the depletion process.

Non-Depleted Protocol

Samples are removed from the AutoPrep and placed in a magnet for 15 minutes. The buffer is aspirated and the cells are Lysed in RLT buffer (Qiagen).

Depleted Protocol

Following AutoPre enrichment, the excess CD45-FITC was removed by washing the sample 2-times with 2 ml of PBS/1% BSA by magnetic separation for 15 minutes. The final sample was resuspended in 900 ul of PBS/5% BSA and then applied to the wells coated with anti-FITC. The samples were incubated for 1 hour with gentle mixing every 15 minutes. After one hour, 300 ul of supernatant was removed from the wells and the cells were placed in a magnet for 15 minutes. The buffer was removed and the cells were lysed in RLT buffer (Qiagen).

For both the depleted and non-depleted samples, the RNA was purified using the Qiagen AllPrep Kit and reverse transcribed using the High Capacity cDNA Reverse Transcription Kits kit (Life Technologies). Complimentary DNA was amplified using TaqMan® PreAmp Master Mix Kit (Life Technologies) and primer sets as for genes from the list in Table 5a. Quantitative PCR was carried out on the amplified samples for two genes known to be expressed in white blood cells (CD45 (aka PTPRC) and BST1) as well as a housekeeping gene expressed (B-Actin). Efficacy of WBC depletion was measured by the loss of WBC and housekeeping specific gene signals in the depleted sample relative to the non-depleted sample.

In FIG. 4 circles and squares represent individual measurements for the on depleted and depleted samples respectively. Lines represent the mean measurement for each sample type. P value represent the results of student's T-test s performed between the depleted and non-depleted samples for each gene measured. T-tests between the depleted and non-deleted samples showed a significant loss of WBC specific and housekeeping genes as a result of CD45 depletion. Mean delta Ct measurements of >2 cycles indicate a >75% reduction of the WBC signals in the CD45 depleted samples.

TABLE 5a Genes tested with CD45 depletion method: Affymetrix Probe Set ID Gene Symbol Gene Title 205648_at WNT2 wingless-type MMTV integration site family member 2 208711_s_at CCND1 cyclin D1 266_s_at CD24 CD24 molecule 201596_x_at KRT18 keratin 18 202589_at TYMS thymidylate synthetase 226553_at TMPRSS2 transmembrane protease, serine 2 225330_at IGF1R insulin-like growth factor 1 receptor 214352_s_at KRAS v-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog 201689_s_at TPD52 tumor protein D52 201005_at CD9 CD9 molecule 224559_at MALAT1 metastasis associated lung adenocarcinoma transcript 1 (non-protein coding) 218638_s_at SPON2 spondin 2, extracellular matrix protein 217776_at RDH11 retinol dehydrogenase 11 (all-trans/9-cis/11-cis) 201656_at ITGA6 integrin, alpha 6 209605_at TST thiosulfate sulfurtransferase (rhodanese) 204623_at TFF3 trefoil factor 3 (intestinal) 202598_at S100A13 S100 calcium binding protein A13 221024_s_at SLC2A10 solute carrier family 2 (facilitated glucose transporter), member 10 226192_at AR androgen receptor 212587_s_at PTPRC protein tyrosine phosphatase, receptor type, C 205715_at BST1 bone marrow stromal cell antigen 1 TMPRSS2 ERG T1E4 gene fusion AKR1C3 aldo-keto reductase family 1, member C3 (3-alpha hydroxysteroid dehydrogenase, type II)

Example 6 Improvement of CTC Detection Following White Blood Cell Depletion

One potential advantage of the CD45 depletion method is an improved ability to detect CTC or characterize CTC gene expression when there is some level of background gene expression contributed by the white blood cells. Reducing the number of white blood cells should reduce the background gene expression and improve the ability to detect transcripts in lower numbers of CTC.

To demonstrate this principle four samples were prepared for each of six donors two samples were spiked with 10 VCaP cells and two samples were unspiked. One spiked and unspiked sample was prepared with both the “Depleted” and “Non-depleted” protocol as described in Example 5. RNA extraction, Reverse Transcription and Pre-Amplification kit were carried out as in example 5. Public databases were mined to identify a panel of potential CTC markers that could be useful for measuring utility of the WBC depletion method. Genes were selected that had moderate to high expression in VCaP cells and a range of expression in WBC. RT-PCR results for potential CTC markers S100A13 and AKR1C3. Without WBC depletion (non-depleted samples) both genes show no significant difference in expression between unspiked and 10-cell spiked sample sets. In the depleted set the background contributed by WBC is reduced and there is a significant difference between the unspiked and 10-cell spike sample sets.

Example 7 Application of WBC Depletion for Molecular Characterization of the CTC Following White Blood Cell Depletion

The major advantage of the CD45 depletion method is enabling enhanced molecular characterization of the CTC and minimizing the background gene expression contributed by the white blood cells.

To demonstrate this application eight samples were prepared from eight donors and two samples each were spiked with 100, 50, 25, and 10 VCaP cells. In addition, four samples containing only VCap cells, for example, 100, 50, 25, and 10 cells were used as positive controls. RNA extraction, Reverse Transcription and Pre-Amplification kit were carried out as in example 6. Androgen Receptor (AR) gene, two AR splice variants (ARV1 and ARV3/7), TMPRSS2 and TMPRSS2:ERG splice variants were selected as the test genes that for VCaP cells. There was a good correlation in the detection of these candidate gene expressions in VCap cells between pure culture VCaP cells and blood spiked with VCap cells after WBC depletion with a correlation coefficient (r2) of >0.9. The results are shown in FIG. 6.

Example 8 Depletion of White Blood Cells Using Bead Based Filtration

This example describes an alternate method for reducing the number of contaminating white blood cells present in samples that have been enriched with the CellSearch Profile Kit. Following EpCAM based immunomagnetic enrichment, contaminating CD45+ WBC are labeled with plastic beads. WBC are separated from CTC by passing the mixture through a filter that retains the bead-bound CD45+ cells and passes the unbound CTC. The CTC are then available for molecular or cellular analysis with a reduced number of contaminating WBC.

To demonstrate the method 10,000 VCAP cells were spiked into six tubes of healthy donor blood and enriched using the Cell Search Profile kit. Three samples were immediately lysed in Qiagen RLT buffer (non-depleted samples) and the remaining three samples were depleted using the bead based method described below. The enriched fraction containing CTC and contaminating white blood cells were mixed with 30 micron plastic beads that had been coated with an antibody specific for CD45 molecule (pluriBead, pluriSelect). The bead/cell mixture was rocked at 8-10 RPM for 1 hour at room temperature. The sample was filtered through a filter with a 27 micron pore size (pluriStrainer, pluriSelect). The filter was washed 2 times and cells collected in the flow through were analyzed by RT-PCR for a white blood cell specific marker PTPRC (aka CD45) and a tumor cell specific marker androgen receptor (AR). Delta Ct values were created by subtracting the expression level (40-Ct value) of the depleted sample from the expression level of the non- depleted sample. A delta Ct of 3.1 cycles for the white blood cell marker PTPRC indicates that the level of white blood cells has been significantly reduced with the bead depletion protocol relative to a non-depleted sample. A delta CT of only 0.4 cycles for the tumor cell specific marker AR indicates that the bead depletion protocol has little impact on the level of tumor specific markers. The results are shown in FIG. 7, which shows the RT-PCR results for white blood cell and tumor cell markers present in samples that have and have not been subjected to the bead depletion procedure. AR=Androgen Receptor PTPRC=Protein tyrosine phosphatase, receptor type, C. 

What is claimed is:
 1. A method of removing white blood cells from a sample comprising enriched rare cells and white blood cells such method comprises (a) treating such sample with a leukocyte marker that is conjugated to a hapten under conditions that label such white blood cells with such leukocyte marker conjugated to a hapten (b) treating the composition of step (a) with a second medium that adheres to such labeled white blood cells, and separating the second medium and its adhered labeled white blood cells from the composition of step (b).
 2. The method of claim 1 wherein the enriched rare cells are selected from the group consisting of CTCs, CECs, CMMCs, and CMCs.
 3. The method of claim 1 wherein the enriched rare cells are selected from the group consisting of CTCs, and CECs.
 4. The method of claim 1 wherein the enriched rare cells are CTCs.
 5. The method of claim 1 wherein such enriched rare cells are labeled with an immunomagnetic marker.
 6. The method of claim 5 wherein the enriched rare cells are CTCs.
 7. The method of claim 1 wherein the leucocyte markers are selected from the group consisting of antibodies to CD 45, CD 19 CD15, glycophorin A, CD2, CD14. CD16, CE38, and CD66b.
 8. The method of claim 1 wherein the leucocyte marker is anti-CD45.
 9. The method of claim 1 wherein the leucocyte marker is an antibody fragment comprising the epitope for CD45.
 10. The method of claim 1 wherein the haptins are selected from the group consisting of fluorescein dye (“FITC”) phycoerythrin (“PE”), allophycocyanin (“APC”) and biotin.
 11. The method of claim 1 where the hapten is FITC.
 12. The method of claim 1 wherein the enriched rare cells are CTCs, the leucocyte marker is anti-CD45, and the haptin in biotin.
 13. A kit for removing white blood cells from a sample comprising enriched rare cells and white blood cells wherein such kit comprises a leukocyte marker that is conjugated to a hapten and a second marker, other than the leukocyte marker wherein the second marker comprises antibodies to such haptens.
 14. A kit for removing white blood cells from a sample comprising enriched rare cells and white blood cells wherein such kit comprises reagents for immunomagnetically marking the rare cells and white blood cells, a leukocyte marker that is conjugated to a hapten and a second marker, other than the leukocyte marker wherein the second marker comprises antibodies to such haptens. 